Mica-insulated spark plug



May 18, 1937. o. c. RoHDE MICA INSULATED SPARK PLUG Filed March 1'7,1936 Patented May 18, 1937 PATENT OFFICE MICA-INSULATED SPARK PLUG OttoC. Rohde, Toledo, Ohio, assigner to Champion Spark Plug Company, Toledo,Ohio, a corporation of Delaware Application March 17, 1936, Serial No.69,345

2 Claims. (Cl. 12S- 169) This invention relates to a mica-insulatedspark plug and has as an object the elimination of corona action, aswill more fully appear' from the detailed description. l

In the accompanying drawing forming a part of this speciiication, thereis shown a longitudinal cross-section of a plug embodying one form .ofthe invention. In this plug there is the usual shell l screw threaded at2 for mounting in the cylinder of an internal combustion engine or thelike. The usual central electrode 3 has its inner end 4 in sparkingrelation with a member 5 mounted on shell 2. The outer end of theelectrode is provided with a suitable connection 6 for l5 receivingelectric current.

Between end 4 and connector 6, the electrode is provided with a sleeve 1which is preferably formed from a thin sheet or sheets of mica woundabout the electrode as a cigarette paper is wound around a cigarette,and commonly referred to in the trade as the mica cigarette, and forbrevity called the cigarette hereinafter.

Surrounding the inner end of the cigarette, above shouldered end 4,there is an insulating member 8, indicated as being formed from micawashers. A holder 9 screw threaded at l0 into the upper end of the shellcontacts the upper end of insulating member 8 and makes a gas tightconnection with the shell by means of a suitable gasket Il. Inside ofholder 9 and between it and cigarette l, therel is an insulating bodyI2, preferably formed of compacted powder such as described in myPatents Nos, 2,020,966 and 2,020,967 and in my co-pending applicationSerial No. 63,- '752, led February 14, 1936. Above shell 9 andinsulating material l2 and below connecting member 6, an insulating bodyI3 surrounds the mica shell. Insulating body I3 is indicated as beingformed of mica washers.

In general construction, this spark plug is quite similar to plugs whichhavebeen in use. However, it has been customary in mica plugs to holdthe central electrode in place and form' a gas tight joint between theholder, electrode and in- 45 termediate insulating material by means ofmetal wedged against the intermediate portion of the mica cigarette, anexample of this construction being shown in my prior Patent No.1,968,314. In mica plugs provided with such metallic holders in contactwith the exterior of the mica cigarette, there is a common phenomenonreferred to as corona which consists in a visible spark or arcapparently under the washers which surround the mica cigarette andcommonly occurring at both ends of the metal contact withtheintermediate portion of the cigarette. This corona action must bedistinguished from flash-over, where the spark jumps across the outsideof the insulation, this latter customarily occurring only when theregular spark gap is wide. The corona action described frequently occurswith even a very narrow spark gap, especially when the plug is operatingunder high pressure. I havediscovered that this corona action resultsfrom the' metallicholder pressed against the outside of the micacigarette, and avoid the corona action by the use of insulation materiall2 in place of metallic holding material pressing against the micacigarette. The compacted insulating powder employed in this way performsthe function of the usual metal holder in firmly gripping the exteriorof the mica cigarette and pressing it against the central electrode,thus` holding the central electrode in proper position, and forming agas tight joint, and at the same time, because of its non-'conductingcharacter, it eliminates the corona action mentioned above and,therefore, forms a more satisfactory plug.

It will be readily understood that departures may be made from the exactform and shape of the parts illustrated, within the scope of theappended claims. y

What I claim is: 1. A spark plug comprising a shell having a `centralholding portion, a central electrode, a

mica cigarette surrounding the electrode, powder compacted between thecigarette and the holding OTTO C.

